The invention relates to a method for producing phthalic anhydride by catalytic gas-phase oxidation of o-xylene and/or naphthalene, wherein the method is carried out by means of a catalyst arrangement which has a first catalyst layer at the gas inlet side and at least one second catalyst layer after the first catalyst layer in the gas flow direction with different catalytic activity, characterized in that when the gas-phase oxidation is being carried out a lower maximum temperature is formed in the first catalyst layer than in the second catalyst layer. The invention furthermore relates to a method for producing the catalyst arrangement according to aspects of the invention, as well as the catalyst arrangement itself according to aspects of the invention.
The industrial-scale production of phthalic anhydride is achieved by the catalytic gas-phase oxidation of o-xylene and/or naphthalene. For this purpose, a catalyst suitable for the reaction is placed in a reactor, preferably a so-called multitube fixed-bed reactor, in which a plurality of tubes are arranged in parallel, and a mixture of the hydrocarbon(s) and an oxygen-containing gas, for example air, is passed through it from top to bottom. Because of the strong heat generation of such oxidation reactions, it is necessary to flush heat-carrier medium around the reaction tubes in order to prevent so-called hotspots and thus to remove the heat energy that has formed. This energy can be used for the production of steam. As a rule, a salt melt, and here preferably a eutectic mixture of NaNO2 and KNO3, is used as heat-carrier medium.
Today, multilayer catalyst beds are used for the oxidation of o-xylene and/or naphthalene to phthalic anhydride. The aim of this is to adjust the activity of the individual catalyst layers to the course of reaction along the reaction axis. It is thereby possible to achieve a high yield of the valuable product PSA and, at the same time, as low as possible a yield of undesired intermediate products such as e.g. maleic anhydride and/or phthalide. Usually, the first catalyst layer (=the catalyst layer placed closest to the reactor inlet) has the lowest activity, as the highest concentration of educts and thus the highest reaction rate occur in the area close to the reactor inlet. Heat being released during the chemical conversion heats the reaction gas up to the point at which the energy generated by the reaction is exactly as great as the energy emitted to the coolant. This hottest point in the reaction tube is called the “hotspot”. Too high an activity in the first catalyst layer will lead to an uncontrolled increase in the hotspot temperature, which can usually lead to a reduction in selectivity or even to a “runaway”.
A further important aspect that must be borne in mind in the design of the activity of the individual catalyst layers is the position of the hotspot in the first catalyst layer. As the catalyst activity reduces as the operating time increases, the position of the hotspot shifts ever further towards the reactor outlet. This can even go so far that the hotspot migrates from the first catalyst layer into the second catalyst layer or even into a layer even further on. Because of the associated significantly reduced PSA yield, in such a case the catalyst needs to be exchanged frequently, which leads to high losses in output.
EP 1 084 115 B1 describes a multilayer catalyst arrangement for the oxidation of o-xylene and/or naphthalene to phthalic anhydride in which the activity of the individual catalyst layers increases continuously from the reactor inlet side to the reactor outlet side. This is achieved by increasing the active material, combined with lowering the alkali metal content of the catalyst such that the catalyst layer directly at the catalyst inlet has the lowest active material content and the highest alkali metal content.
DE 103 23 818 A1 describes a multilayer catalyst arrangement for the oxidation of o-xylene and/or naphthalene to phthalic anhydride, made of at least three successive catalyst layers in which the activity of the individual catalyst layers increases continuously from the reactor inlet side to the reactor outlet side. This is achieved by using TiO2 with different BET surface areas such that the BET surface area of the TiO2 used is smaller in the catalyst layer at the reactor inlet than in the following catalyst layers and is at its largest in the last catalyst layer (reactor outlet).
DE 103 23 461 A1 describes a multilayer catalyst arrangement for the oxidation of o-xylene and/or naphthalene to phthalic anhydride in which the activity of the individual catalyst layers increases from the reactor inlet side to the reactor outlet side, wherein the ratio of V2O5 to Sb2O3 in the first catalyst layer is between 3.5:1 and 5:1.
DE 103 23 817 A1 describes a multilayer catalyst arrangement for the oxidation of o-xylene and/or naphthalene to phthalic anhydride, made of at least three successive catalyst layers in which the activity of the individual catalyst layers increases continuously from the reactor inlet side to the reactor outlet side, wherein the last layer, lying closest to the reactor outlet, contains more than 10 wt.-% V2O5 and has phosphorus as the only layer.
A disadvantage of the catalysts or multilayer catalyst systems according to the invention indicated there is that, despite the use of such structured catalysts, the life of the catalyst is not satisfactory, in particular with regard to the increasing shift of the hotspot in the direction of the gas flow. A positioning of the hotspot in the most active catalyst layer further towards the gas outlet side also limits the possibility of finely adjusting the selectivity of the catalyst to reduce undesired by-products.